Implant Improving Local Bone Formation

ABSTRACT

A bone implant comprises an active agent on at least a portion thereof. The active agent is locally deliverable to bone proximate the implant in at least a two-phased release scheme. A first phase rapidly releases a first quantity of the active agent, and at least a second phase gradually releases a second quantity of the active agent, whereby bone formation stimulated by the active agent is modulated. In one embodiment, a porous implant comprises a porous portion coated with a calcium phosphate compound and which is contacted with a bisphosphonate compound to form a bisphosphonate layer chemically bound to the calcium phosphate at the surface of the porous portion and to form bisphosphonate molecules being non-chemically attached inside the pores of the porous portion. The non-chemically attached bisphosphonate molecules are released in the subject at a rate greater than that of the chemically bound bisphosphonate layer.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

This invention relates to a bone implant, and more particularly to animplant improving local bone formation around and/or within the implant.

(b) Description of Prior Art

Bone growth into porous materials has proven to be a very effectivemethod for attaching prosthetic implants to the bony skeleton (Engh C A,Claus A M, Hopper R H, Engh C A. Clin Orthop 393:137-146, 2001; TelokenM A, Bissett G, Hozack W J, Sharkey P F, Rothman R H. J Bone Joint Surg[Am] 84-A:2140-2144, 2002; D'Antonio J A, Capello W N, Manley M T,Geesink R. Clin Orthop 393:101-111, 2001; Pidhorz L E, Urban R M, JacobsJ J, Sumner D R, Galante J O. J Arthrop 8:213-225, 1993; Sychterz C J,Claus A M, Engh C A. Clin Orthop 405:79-91, 2002). However, thereremains a need to develop modalities that can accelerate and/or increasebiologic fixation. The more rapid and the greater the amount of boneformation around and/or within an implant, the faster the implantbecomes mechanically secured against the disruptive forces of loadbearing and the sooner patients can safely return to their activities ofdaily living. In situations where bone stock is frequently compromised,or where initial implant stability is more tenuous (such as in theelderly, post-traumatic cases, or revision surgery), both short andlong-term clinical results are inferior, and the construct would clearlybenefit from enhanced biologic fixation. As well, the more extensive theperi-implant tissue formation, the more protected is the bone-implantinterface against wear particle induced periprosthetic osteolysis (BobynJ D, Jacobs J J, Tanzer M, Urban R M, Aribindi R, Sumner R, Turner T,Brooks C E, Galante J O: Clin Orthop 311:21, 1995). Increasedperi-implant bone formation may also minimize the risk of postoperativeperiprosthetic fractures and provide additional bone stock if asubsequent revision is needed. An additional issue relates to thebone-implant interface in the immediate post-operative phase. A likelyscenario for the onset of prosthetic loosening is that initial implantfixation is compromised by the resorption of the traumatized andnecrotic bone adjacent to the implant. This theory is supported by thequantitative radiostereometry studies of Ryd et al (Ryd L, Albrektsson BE, Carlsson L, et al: J Bone Joint Surg [Br] 77:377-83, 1995) thatshowed postoperative implant migration predicts later loosening. Thisearly migration must be related to bone resorption, since oralbisphosphonate therapy has recently been shown to reduce the initialmigration of knee prostheses through its inhibitory effect onosteoclastic function (Hilding M, Ryd L, Toksvig-Larsen S, Aspenberg P:Acta Orthop Scand 71:553-7, 2000).

Various methods have been investigated to increase the rate and/or theextent of bone growth into porous implants, with varying degrees ofsuccess. Due largely to practical limitations and/or cost issues, onlycalcium phosphate coatings, and most notably hydroxyapatite, have todate reached the point of clinical applications. (Geesink R. Clin Orthop225:147-170, 1990; Bauer T W, Geesink R C, Zimmerman R, McMahon J T. JBone Joint Surg [Am] 73:1439-1452, 1991; D'Antonio J A, Capello W N,Manley M T, Geesink R. Clin Orthop 393:101-111, 2001; Overgaard S,Bromose U, Lind M, Bunger C, Soballe K. J Bone Joint Surg [Br]81:725-731, 1999) Of particular recent interest is the use ofbisphosphonates for modifying bone remodeling around orthopaedicdevices. Bisphosphonates selectively absorb to bone mineral and inhibitbone resorption by interfering with the action of osteoclasts. It isbelieved that bisphosphonates are internalized by osteoclasts, interferewith specific biochemical processes and induce apoptosis. Allbisphosphonates contain two phosphonate groups attached to a singlecarbon atom, forming a P—C—P structure; as such they are stableanalogues of naturally occurring pyrophosphate-containing compounds. Themore potent nitrogen-containing bisphosphonates, such as zoledronic acid(ZA), may affect cellular activity and cell survival by interfering withprotein prenylation and therefore the signaling functions of keyregulatory proteins (Russell R G G, Rogers M J: Bone 25:97-106, 1999).

Recent literature has described the utility of bisphosphonates foraffecting the osteoblastic/osteoclastic cellular response in both matureand healing bone (Green JR, Müller K, Jaeggi K A. J Bone Miner Res9:745-751, 1994; Pataki A, Muller K, Green J R, Ma Y F, Li Q N, Jee W S.Anat Rec 249:458-468, 1997). This has resulted in oral bisphosphonatetherapy for helping to mitigate the osteolytic effects of accumulatedwear debris around joint replacement implants (Shanbhag A S, Hasselman CT, Rubash H E. Clin Orthop 344:33-43, 1997; Shanbhag A S, May D, Cha C,Kovach C, Hasselman C T, Rubash H E. Trans Orthop Res Soc 24:255, 1999;Horowitz, S M, Algan, S A, Purdon M A. J Biomed Mater Res 31:91-96,1996.) As well, bisphosphonates have been used to manage periprostheticbone loss as might occur through stress shielding mechanisms(Soininvaara T A, Jurvelin J S, Miettinen H J A, Suomalainen O T, AlhavaE M, Kroger P J. Calcified Tissue Int 71:472-477, 2002; Venesmaa P K,Kroger H P, Miettinen H J, Jurvelin J S, Suomalainen O T, Alhava E M. JBone Miner Res 16:2126-2131, 2001; Wilkinson J M, Stockley I, Peel N F,Hamer A J, Elson R A, Barrington N A, Eastell R. J Bone Miner Res16:556-564, 2001). Also of important note is that Hilding et al (HildingM, Ryd L, Toksvig-Larsen S, Aspenberg P: Acta Orthop Scand 71:553-7,2000) showed an early postoperative oral regimen of clodronate reducedmigration of knee prostheses, as measured by radiostereometry. Inexperimental rabbit studies, Little et al (Little D G, Cornell M S,Briody J, Cowell C T, Arbuckle S, Cooke-Yarborough C M. J Bone JointSurg [Br] 83-B:1069-1074, 2001) have shown that in distractionosteogenesis a single postoperative intravenous dose of pamidronate (3mg/kg) decreased the disuse osteopenia normally associated withlengthening and increased the amount and density of the regenerate bone.In a further study, Little et al (Little D G, Smith N C, Williams P,Briody J, Bilston, L, Smith E J, Gardiner E M, Cowell C T. J Bone MinerRes 18:1300-1307, 2003) showed that one or two doses of the more potentZA abolished osteopenia and increased regenerate volume, mineralizationand strength.

There has been speculation about the possibility of bisphosphonatesacting not only to suppress osteoclastic activity but also to stimulateosteoblastic activity (Green J R, Muller K, Jaeggi K A. J Bone Miner Res9:745-751, 1994; Pataki A, Müller K, Green J R, Ma Y F, Li Q N, Jee W S.Anat Rec 249:458-468, 1997; Shanbhag A S, Kenney J, Manning C, FlanneryM, Rubash H, Harris W, Goldring S. Trans Orthop Res Soc 25:688, 2000).However, recent findings by Smith et al (Smith E J, Bugler R J, Peat RA, McEvoy A, Briody J N, Baldock P A, Eisman J A, Little D G, Gardiner EM. Trans Orthop Res Soc 28:351, 2003) have shown that the increase innet bone accumulation from ZA were due to an increase in retention ofcallus; the bone formation rate was actually reduced. Modulation of boneturnover shifted the balance of formation and resorption in a favorablemanner resulting in a net increase in total regenerate at six weeks.Remodeling took place over 45 weeks in this model, indicating that theeffects of ZA are long lasting.

On a cost basis alone, bisphosphonates could have a substantialadvantage over recombinant proteins for improving the bone healingresponse within and around orthopaedic implants. Although intravenousdelivery of ZA is both feasible and convenient, it subjects the patientto various systemic and potentially adverse effects.

Local delivery of medicinal products for implants of various types hasbeen attempted, however with varying degrees of success. For example,anti-inflammatory agents delivered to coronary stent implant sites havebeen shown to increase patency rates. Local delivery of bisphosphonatesin dental surgical applications has also recently been attempted. Forexample, local application of alendronate, a second generationbisphosphonate, following periodontal surgery in rats has been shown toreduce alveolar bone resorption. (Binderman I, Adut M, Yaffe A: JPeriodontol 71:1236-40, 2000; Yaffe A, lztkovich M, Earon Y, Alt I,Lilov R, Binderman: J Periodontol 68:884-9, 1997) Another rat study hasfurther shown that a single application of alendronate via a sponge toan implant site reduces the amount of soft tissue that forms as aconsequence of resorptive remodeling from repetitive implant motion(Astrand J, Aspenberg P: J Orthop Res 22:244-249, 2004).

The concept of immobilizing a bisphosphonate compound via hydroxyapatitehas previously been explored for dental surgical applications,particularly in the context of smooth surface tooth root implants.(Meraw S J and Reeve C M., Qualitative analysis of peripheralpen-implant bone and influence of alendronate sodium on early boneregeneration, J Peridontology 70:1228-1233, 1999; Yoshinari M, et al.,Bone response to calcium phosphate-coated and bisphosphonate-immobilizedtitanium implants, Biomaterials 23:2879-2885, 2002; Ganguli A, et al,The interaction of bisphosphonates in solution and as coatings onhydroxyapatite with osteoblasts, J Mater Sci: Mater Med 13:923-931,2002; Denissen H, et al, Normal osteoconduction and repair in and aroundsubmerged highly bisphosphonate-complexed hydroxyapatite implants in rattibiae, J Periodontology 71:272-278, 2000). These studies have shownthat local release of bisphosphonate compounds, bound to dental implantsthrough an intermediary hydroxyapatite coating, results in a net gain inperi-implant bone formation. However, the extent of bone formationaround the bisphosphonate coated implants shown by these studies remainsrelatively low compared to the control specimens. Therefore, howeverpositive, relatively limited benefits with respect to bone formationhave resulted.

Therefore, while the local delivery of bisphosphonate to an implant sitefor improving the bone healing response within and around the implant isdesirable, the means by which the medicinal compounds are locallyadministered poses challenges for many implant applications. As such, aneed exists for an improved means and method for administering a localrelease of bisphosphonate to allow the compound to positively affectperi-implant bone remodeling, while avoiding the systemic exposure. .Further, a need exists to improve the extent of bone formation by suchlocally released bisphosphonate.

It would therefore be highly desirable to be provided with a new implantimproving bone formation around and/or within the implant.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention there is provideda porous implant comprising a porous portion coated with a calciumphosphate compound, said implant having been contacted with abisphosphonate compound to form a bisphosphonate layer chemically boundto said calcium phosphate compound at the surface of said portion andbisphosphonate molecules being non-chemically attached inside pores ofsaid portion, said non-chemically attached bisphosphonate moleculesbeing burst-releasable in a subject upon contact with body fluids andsaid chemically bound bisphosphonate layer being slowly releasable insaid subject upon contact with said body fluids.

In accordance with another aspect of the present invention, there isalso provided a porous bone implant comprising a porous portion coatedwith a calcium phosphate compound on an outer surface thereof and havinga bisphosphonate compound applied to said porous portion to form abisphosphonate layer chemically bound to said calcium phosphate on saidouter surface of said porous portion, said bisphosphonate layer beingreleasable from the implant to promote bone formation around and/orwithin said implant when implanted in said subject.

There is also provided, in accordance with another aspect of the presentinvention, a biocompatible bone implant comprising a bone growthstimulating portion having at least a first region with a calciumphosphate coating thereon and at least a second region free of saidcalcium phosphate, said bone growth stimulating portion having abisphosphonate compound applied thereto to form a bisphosphonate layerchemically bound to said calcium phosphate over said first region andbisphosphonate molecules being non-chemically attached to said bonegrowth stimulating portion over said second region, wherein saidbisphosphonate compound is released from said first and second regionsat different rates when said implant is installed within a subject.

There is further provided, in accordance with another aspect of thepresent invention, a biocompatible bone implant comprising a bone growthstimulating active agent on at least a portion thereof, said activeagent being locally deliverable to bone proximate said implant in atleast a two-phased release scheme, wherein a first phase rapidlyreleases a first quantity of said active agent and at least a secondphase gradually releases a second quantity of said active agent, wherebybone formation stimulated by said active agent is modulated.

In one embodiment of the present invention, the implant is forreplacement of a joint such as, but not limited to hip, knee, elbow,ankle and shoulder. In an alternate embodiment, the implant includes aspine or dental implant.

In one embodiment of the present invention, the implant is entirelyporous.

In a preferred embodiment of the present invention, the implant includesat least a porous portion which comprises a biocompatible surface havinginterconnecting pores formed therein. Alternatively, the biocompatiblesurface may be a sintered bead porous surface, a fiber metal poroussurface, a textured surface, a plasma spray surface, or any other typeof surface that one skilled in the art would envision as being suitablefor the purpose of the present invention.

In one embodiment of the present invention, the implant is made from amaterial comprising at least one of titanium, titanium-based alloy,zirconium, niobium, cobalt-based alloy, tantalum and polymer composite.In a preferred embodiment of the present invention, wherein the implantincludes a porous portion, the pores in the implant are of a sizeranging from about 20 to about 1000 μm, more preferably the pores are ofan average size of about 100 to about 700 μm.

In a preferred embodiment of the present invention, the bisphosphonateis at least a third generation bisphosphonate, such as one ofbisphosphonate zoledronic acid (ZA), ibandronate and risedronate, forexample. Most preferably, the selected third generation bisphosphonateis bisphosphonate zoledronic acid (ZA). While it is to be understoodthat the present invention may employ any bisphosphonate compound, atleast third generation bisphosphonate compounds (i.e. third generationor later) are preferably used due to their improved potency over earliergenerations. However, examples of first and second generationbisphosphonates which can nonetheless be used, however may be lesseffective, include: etidronate; clodronate; tiludronate; pamidronate;dimethyl pamidronate; and alendronate. It is also contemplated thatsubsequent generations of bisphosphonate compounds, having furtherimproved potency, may also be employed as they become available.

The term “bisphosphonate of the third generation” or “third generationbisphosphonate” is widely used and accepted in the literature, andtherefore one skilled in the art will understand which bisphosphonatecompounds constitute third generation compounds. For example, such thirdgeneration bisphosphonates include zolendronate (zoledronic acid),risendronate and ibandronate.

All references herein are hereby incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a photograph of an HA-coated porous tantalum ulnar implantdosed with 0.05 mg zoledronic acid.

FIG. 1B is a scanning electron micrograph of the HA-coated pores.

FIG. 1C is a backscattered scanning electron micrograph illustrating theHA coating on the superficial tantalum struts. The tantalum strutsappear as white in the backscattered images, HA as light grey.

FIG. 1D is a backscattered scanning electron micrograph illustrating theHA coating (see arrows) on the superficial tantalum struts. The tantalumstruts appear as white in the backscattered images, HA as light grey.

FIG. 2 is a cross-sectional view of an embodiment of the presentinvention where the porous portion may be a porous material of asintered bead surface for example, wherein the binding agent and theactive agent applied thereto are schematically depicted.

FIG. 3 is a cross-sectional view of an alternative embodiment of thepresent invention where the porous portion is made of fiber metal.

FIG. 4 is a cross-sectional view of an alternative embodiment of thepresent invention where the porous portion is made of a texturedsurface.

FIG. 5. illustrates spindle fixture used for manual dosing of implantswith zoledronic acid in aqueous solution with dosing pipette and vial ofZometa™.

FIG. 6. is a post-mortem contact radiograph illustrating position of anintramedullary implant within the ulna.

FIG. 7. illustrates elution curves of the percent release in saline ofzoledronic acid from non-HA-coated and HA-coated implants dosed with0.05 mg zoledronic acid. Data points at each time period represent themean of 4 soaked implants with standard deviation. Note the rapid andcomplete release of zoledronic acid on implants without HA coatingcompared to those with HA coating.

FIG. 8. is a representative contact radiographs of serial transversehistologic sections. Control (left) and ZA-dosed (right) implants aredepicted extending from the metaphyseal (top) to diaphyseal (bottom)regions of the ulna. Additional bone and/or greater peri-implant bonedensity is visible in the ZA-dosed sections compared with controls.

FIG. 9. is a representative contact radiographs (top) with correspondingbackscattered scanning electron micrographs (bottom) of control (left)and ZA-dosed (right) implant sections taken in the metaphysis. Note theadditional bone filling the intramedullary canal in the ZA-dosedsection.

FIG. 10. is a representative contact radiographs (top) withcorresponding backscattered scanning electron micrographs (bottom) ofcontrol (left) and ZA-dosed (right) implant sections taken in thediaphysis. Additional bone is clearly visible in the ZA-dosed section.

FIG. 11. is a backscattered scanning electron micrographs of a controlsection (left, 12.5% ingrowth) and a ZA-dosed section (right, 19.8%ingrowth) selected for the extent of bone ingrowth preciselycorresponding to the mean for each group. The implant and surroundingbone were digitally subtracted from the images to clarify theillustration. The implant diameter is 5 mm. Note the tendency for morebone at the periphery where the implants are HA-coated but the presenceof bone islands throughout the cross-sections.

DETAILED DESCRIPTION OF THE INVENTION

The hypothesis that bisphosphonates have a positive effect on net bonegrowth, particularly into porous implants, was first confirmed byconducting a canine ulnar intramedullary implant model in which poroustantalum rods were implanted for 6 weeks. (Bobyn J D, Tanzer M, Harvey EJ, Krygier J J, Little D G. J Bone Joint Surg (Br), 2005) Immediatelyafter surgery, seven test animals with 14 ulnar implants wereadministered a single intravenous dose of 0.1 mg/kg ZA (Novartis PharmaA G, Basel, Switzerland). Because of the systemic exposure of testanimals to ZA it was necessary to utilize external control data from aprior experiment (Tanzer M, Kantor S, Bobyn J D. J Arthrop 19:195-199,2002) for comparisons of bone ingrowth. The mean extent of bone ingrowthwas 6.6% for the control implants and 12.2% for the ZA-treated implants,a relative difference of 85% that was statistically significant.Detailed quantitative analysis of the individual islands of new boneformation with the implant pores revealed that the number of boneislands was similar for both implant groups but the average bone islandsize was 69% larger in the ZA-treated group. This finding was consistentwith the documented suppression of osteoclastic remodeling withbisphosphonate therapy (Day J, Ding M, Bednarz P, Van Der Linden J,Mashiba T, Hirano T, Johnston C, Burr D, Hvid I, Sumner D, Weinans H.Trans 48^(th) Orthop Res Soc 27:85, 2002) and the study of Smith et al(Smith E J, Bugler R J, Peat R A, McEvoy A, Briody J N, Baldock P A,Eisman J A, Little D G, Gardiner E M. Trans Orthop Res Soc 28:351, 2003)suggesting that osteoblastic activity is not increased in the presenceof bisphosphonates.

Thus, delivery of a bone stimulating active agent such as bisphosphonateto an implant site for improving the bone healing response within andaround the implant is effective. The preferred embodiments of thepresent invention as described below further provide an improved localdelivery means of the bisphosphonate active agent, enabling a modulateddelivery of the active agent from the implant which promotes boneformation around and/or into the implant. More particularly, the presentinvention preferably provides at least a biphasic release of thebisphosphonate compound.

Referring to FIGS. 1 a-4, an implant 10 comprises at least a bone growthstimulating portion 12 around and/or within which bone proximate theretois capable of growing such that the surrounding bone fuses with theimplant. The implant 10 preferably includes the bone growth stimulatingportion 12 on at least a portion of an outer surface thereof which isadapted to be proximate to a bone surface when the implant 10 isinstalled within the receiving subject, whether patient, test animal orcadaver. Preferably, the bone growth stimulating portion 12 covers thecomplete outer surface of the implant 10, however the implant 10 mayinclude discrete bone growth stimulating portions 12 only on selectedportions thereof. Alternately, and in one preferred embodiment asdepicted in FIG. 1, the implant 10 may be completely composed of such abone growth stimulating portion 12. Preferably, the bone growthstimulating portion 12 is porous, having a plurality of interconnectingpores 16 formed therein. As such the bone growth stimulating portion 12will be referred to herein as a porous portion 12 in accordance with aembodiment, however it is to be understood that other suitable materialstructures maybe used which are biocompatible and upon and around whichbone may grow.

A layer of binding agent 14 is applied to at least the outer surface 13of the porous bone growth stimulating portion 12 of the implant 10,thereby coating this outer surface 13 thereof. The binding agent 14 isone which has an affinity to bone for engagement therewith, and withwhich the active agent will chemically bind. Preferably, the bindingagent 14 comprises a calcium phosphate compound and the active agentcomprises a bisphosphonate which will chemically bind to the calciumphosphate substrate layer. The term chemically bound as used herein isintended to include covalent and ionic bonds which may form to removablyengage the active agent to the binding agent. In the embodimentdescribed in greater detail below, the calcium phosphate binding agent14 comprises hydroxyapatite (HA), however other calcium phosphateformulations may also be used, such as tricalcium phosphate for example,or any mixture of tricalcium phosphate and HA. Although HA is used inthe examples below, both HA and tricalcium phosphate are known calciumphosphates employed in various biomedical applications, particularly asbiocompatible coatings on dental and hip implants.

Due to the porous nature of the bone growth stimulating portion 12, thecalcium phosphate binding agent 14 applied thereto preferably covers theouter surface 13 of the implant, while the internal surfaces of pores 16within the portion 12 remain substantially uncoated. However, dependingon the method employed to apply the calcium phosphate compound to theimplant, a quantity calcium phosphate may enter the inner pores 16within the implant. The majority of the calcium phosphate compoundapplied, however, will tend to form a layer on the outer surface of theimplant. Various methods may be used to apply the calcium phosphatecompound to the implant may be used, such as chemical deposition andplasma spray deposition for example, which are well known in the art. Inthe embodiment described below, plasma spray deposition is used to applythe calcium phosphate to the implant. As this application techniquerequires physically spraying a liquid based form of the calciumphosphate directly onto the implant, mainly the outer surface thereof iscoated, leaving the inner pores relatively free of the calcium phosphatecompound.

The porous portion 12 of the implant 10 having the calcium phosphatebinding agent applied thereto is then contacted with an active agent toform a layer 24 of said active agent which chemically binds to thebinding agent 14 on the outer surface 13 of the porous portion 12. Theactive agent acts to stimulate bone formation when released to thesurrounding bone within which the implant is disposed. In the preferredembodiment of the present invention, the active agent employed is abisphosphonate compound which chemically binds to the calcium phosphate.The active agent is applied to the implant such that it coats at least amajority of the bone growth stimulating portion 12 thereof, both thosewith and without the binding agent thereon. Thus, the active agentattaches to the implant over regions which are substantially free of thebinding agent, as well as those which have the binding agent thereon. Inthe preferred embodiment, the bisphosphonate molecules which areprovided within the pores 16 become physically attached (i.e.non-chemically bound) to the porous surfaces below the outer surface ofthe implant due to the relative absence of the calcium phosphatethereon. The release rate of the bisphosphonate chemically bound to thecalcium phosphate from the implant 10 to a surrounding bone structurediffers from that of the non-chemically bonded bisphosphonate. Thisaccordingly provides a biphasic elution profile of the bisphosphonatecompound from the implant 10, as will be described further below.

In a preferred embodiment, a porous implant 10 promoting bone formationaround and/or within the implant when implanted in a subject, theimplant 10 comprising a porous portion 12 coated with a calciumphosphate 14, such as hydroxyapatite, having been contacted with abisphosphonate compound, preferably a third generation bisphosphonatesuch as zoledronic acid, to form a bisphosphonate layer chemically boundto the hydroxyapatite on at least a partial region of said porousportion 12.

Preferably the physical structure of the implant, or at least the bonegrowth stimulating portion thereof, allows a first region (for examplethe outer surface) to be coated with a binding agent such as calciumphosphate while at least a second region thereof (for example the innersurfaces of the pores formed therein) remains substantially free of thebinding agent. Thus, once by the first and second regions are coated byan active agent, such as a bisphosphonate, the active agent will releasefrom each of the first and second regions of the implant at differentrelease rates. However, it nevertheless remains possible to achieve sucha biphasic release rate, or even a multi-phasic release rate of theactive agent, by alternately providing different binding agents to thefirst and second regions. For example, a first calcium phosphateformulation is applied to a first region of the bone implant and asecond, different, calcium phosphate formulation is applied to a secondregion of the bone implant. Both the first and second regions are coatedwith a bisphosphonate active agent, which chemically bonds to each ofthe calcium phosphate binding agents. However, the release rate of thebisphosphonate active agent from each of the two calcium phosphatebinding agents will be different, thus providing a biphasic release ofthe active bisphosphonate to the surrounding bone. The present inventiontherefore includes such alternate means of achieving local, multi-phasicrelease of the bone formation stimulating active agent from the implantto the surrounding bone in order to promote and modulate bone formation.

Although the bone growth stimulating portion 12 is preferably porous,other suitable bone growth stimulating structures may also be used,providing such structures are biocompatible and permit bone formationaround and/or within this region of the implant. Particularly, suchstructures enable portions thereof to be coated with a binding agent,such as a calcium phosphate and more preferably such as hydroxyapatite,while others remain uncoated. For example, a surface having groovesrather than pores, or alternately having other forms of surface featuressuch as depressions and/or raised portions, may also be used. Further,while such structural surface and/or material features enable apractical means of coating only certain regions of the implant withhydroxyapatite while others remain uncoated, it is to be understood thata similar effect may be achieved on a relatively level outer surface ofthe implant by selectively applying the hydroxyapatite to predeterminedsurface regions thereof, while other surrounding regions remain bare.This may be done, for example, by masking regions of the implant not tobe covered by binding agent prior to the application thereof.

The biocompatible surface of the bone growth stimulating portion mayalso be a sintered bead porous surface 18 as shown in FIG. 2, a fibermetal porous surface 20 as shown in FIG. 3, a textured surface 22 asshown in FIG. 4, a plasma spray surface or any other type of surfacethat one skilled in the art would envision as being suitable for thepurpose of the present invention.

A preferred embodiment of the present invention is described below.

Materials and Methods

Binding of Bisphosphonate. A simple and effective method for binding abisphosphonate to an orthopaedic implant involves coating it with a thinlayer of a calcium phosphate such as hydroxyapatite and depositing thebisphosphonate in aqueous solution directly onto the implant. Thistechnique takes advantage of the known chemical affinity ofbisphosphonates for calcium phosphate, the same affinity that enablestheir selective binding to bone.

Rationale for Zoledronic Acid. Zoledronic acid (ZA), or zolendronate, isa 3rd generation bisphosphonate compound which was utilized for allstudies. ZA is structurally similar to other bisphosphonates, having therequired phosphorus-carbon-phosphorus structure, as shown below.

Third generation bisphosphonates generally include a hydroxyl group,associated with enhanced binding to bone, which is present in the R¹position. An additional imadazole group containing two nitrogens at theR² position distinguishes ZA from other bisphosphonates. Thissubstituent at the R² makes ZA acid 100 and 250 times more potent than2nd generation compounds such as pamidronate and alendronate,respectively (Green, J R. Results of comparative preclinical studies.Seminars in Oncology 28:4-10, 2001; Li E C and Davis L E. ClinicalTherapeutics 25:2669-2708, 2003). Table 1 below list examples ofbisphosphonates, identified by generation, with their R¹ and R²components.

TABLE 1 Bisphosphonate Generation R¹ R² Etidronate 1^(st) OH CH₃Clodronate 1^(st) Cl Cl Tiludronate 1^(st) H CH₂SPhenyl-Cl Pamidronate2^(nd) OH CH₂CH₂NH₂ Dimethyl 2^(nd) OH CH₂CH₂N(CH₃)₂ pamidronateAlendronate 2^(nd) OH CH₂CH₂CH₂NH₂ Ibandronate 3^(rd) OHCH₂CH₂N(CH₃)(Pentyl) Risendronate 3^(rd) OH CH₂-3-pyridine Zolendronate3^(rd) OH CH₂(1H-imidazole-1-yl)

The potency of bisphosphonates in inhibiting bone resorption appears tobe dependent largely on the R2 side chain. In particular, thosebisphosphonates containing a nitrogen atom at a critical distance fromthe P—C—P group and in specific spatial configuration are considerablymore potent than non-nitrogen containing bisphosphonates. For example,second generation bisphosphonates such as pamidronate and alendronatethat contain a basic primary nitrogen atom in an alkyl chain, areapproximately 10-100 fold more potent than first generationbisphosphonates such as etidronate or clodronate. Third generationbisphosphonates which contain a tertiary nitrogen, such as ibandronate,risendronate and zoledronate, are even more potent at inhibiting boneresorption. Replacement of one phosphate group with a carboxylate groupor methylation of phosphonate by replacement of a hydroxyl group resultsin similar affinity for bone, although very different anti-resorptivepotencies. Thus, the two phosphonate groups are required both fortargeting to bone and for the molecular mechanism of action. Althoughthe bisphosphonate most preferably used in the present invention is ZA,another third generation bisphosphonate can also be used. While lesseffective, bisphosphonate compounds of earlier generations may also beemployed, however it is understood that results may be less marked.Further, it is to be understood that subsequent, more potent,generations of bisphosphonates may also be employed in accordance withthe present invention as these become available.

Rationale for Implant Material. A porous tantalum biomaterial wasutilized for the in vivo studies (FIG. 1). The rationale for thisselection was multi-factorial. The 80% volume porosity of poroustantalum provides a very open structure into which bone can heal,virtually unimpeded by the material, without the need for a solidsubstrate. The stiffness of porous tantalum is very low, ˜3 GPa, withinthe stiffness of range of bone and 5-10 times less stiff than otherporous coatings such as titanium fiber metal sintered beads. For in vivostudies this is important because it minimizes any stress shieldingeffects the implant might have on bone healing and remodeling, thusmaximizing the ability to detect the effect of bisphosphonate release onbone ingrowth/remodelling. The porous tantalum material has beencharacterized for its bone ingrowth characteristics in prior implantmodels (Bobyn J D, Tanzer M, Harvey E J, Krygier J J, Little D G. J BoneJoint Surg (Br), In press, 2005; Bobyn J D et al: J Bone Joint Surg[Br]81:907-914, 1999; Bobyn J D, Toh K-K, Hacking S A, Tanzer M, KrygierJ J: J Arthrop 14:347-354, 1999).

As noted above, although tantalum was used in one embodiment, theimplant may be made from a material comprising at least one of titanium,titanium-based alloy, zirconium, niobium, cobalt-based alloy, tantalum,and a polymer composite.

Implants 5 mm in diameter and 50 mm in length (FIG. 1) were manufacturedfor use in a canine ulnar intramedullary model.

Rationale for Hydroxyapatite Coating. Hydroxyapatite (HA) coatingsdeposited by plasma spray techniques have been successfully utilized injoint replacement implants for almost two decades (Geesink R. ClinOrthop 261:39-58, 1990; Bauer T W, Geesink R C, Zimmerman R, McMahon JT. J Bone Joint Surg [Am] 73:1439-1452, 1991; D'Antonio J A, Capello WN, Manley M T, Geesink R. Clin Orthop 393:101-111, 2001; Overgaard S,Bromose U, Lind M, Bunger C, Soballe K. J Bone Joint Surg [Br]81:725-731, 1999). Although the typical thickness of HA coatings usedclinically is ˜50 micrometers, a thinner coating of 10-15 micrometerswas utilized so there was less occlusion or alteration of the pore sizeand pore interconnectivity. Because the plasma spray technique isline-of-sight, only the superficial struts comprising the poroustantalum structure were coated (FIG. 1). The HA coating was commerciallyapplied in the same manner as clinically used coatings (but thinner).The methodology of applying HA coating was highly reproducible since itwas computer controlled and involved identical coating amount for eachimplant. The final chemistry of the HA coating was well controlledthereby ensuring uniformity of ZA deposition and binding from implant toimplant. HA specifications were 98% HA, 99% dense, 64% crystalline and acalcium:phosphate ratio of 1.67. Other calcium phosphates may also beused, and will have parameters with respect to density, crystalinity,etc which differ from those of HA.

As noted above, HA is but one calcium phosphate compound which may beused. Examples of other calcium phosphates which are of biomaterialsinterest and may be used are listed in Table 1.1 below. It is understoodthat any mixtures of these calcium phosphates may also be used.Amorphous calcium phosphate is a phase which is often formed during hightemperature processing, such as when plasma spraying hydroxyapatite.Hydroxyapatite is the least soluble of the calcium phosphates listed,and is the most stable at pH's above 4.2. Therefore, under normalphysiological conditions of pH 7.2, hydroxyapatite is preferred. Theterm hydroxyapatite as used herein is intended to refer to the calciumphosphate compound pentacalcium hydroxyl apatite identified on the tablebelow.

TABLE 1.1 Chemical Chemical Name Abbr Formula Phase Ca:P Amorphous ACP —— — calcium phosphate Dicalcium DCP CaHPO₄ Monetite 1.00 PhosphateTricalcium α-TCP Ca₃(PO₄)₂ 1.50 Phosphate Tricalcium β-TCP Ca₃(PO₄)₂Whitlockite 1.50 Phosphate Pentacalcium HAp Ca₁₀(PO₄)₆(OH)₂Hydroxyapatite 1.67 Hydroxyl Apatite Tetracalcium TTCP Ca₄O(PO₄)₂Hilgenstockite 2.00 Phosphate Monoxide

Method for Applying/Binding ZA to Porous Tantalum Implants. ZA wasobtained through our hospital pharmacy under the trade name Zometa™(Novartis, Basel, Switzerland). Zometa™ is packaged in vials of powderthat contain 4.0 mg ZA, 220 mg mannitol (a sugar bulking agent), and 24mg sodium citrate (a buffer). The powder was mixed with distilled waterand an aliquot representing a specified amount of ZA was collected in amicropipette. The micropipette was used to deposit the solution over thelength of each of the small (5×50 mm) porous tantalum implants. This wasdone in a systematic manner, with the implants held at its ends in aspindle, and equal sized drops of solution manually deposited at 0.5 cmintervals along the implant length and 90 degree intervals around itscircumference (FIG. 5). The concentration of the ZA in solution wasadjusted so that about 0.5 ml of solution was required/utilized fordeposition over the entire surface of the porous tantalum implant. Thiswas sufficient volume to ensure that the entire surface of the implantwas easily saturated with liquid thus helping to create a uniformdeposition of the drug on the implant. This technique resulted indeposit of the ZA solution on and within the entire implant, not just onthe superficial region containing HA. The implant was subsequently driedin an oven at 50° C. for 24 hours to ensure fluid evaporation. Theresult was an implant with some ZA bound chemically to the HA coatingand some ZA left on the inner, non HA-coated tantalum struts, presumablyunbound and available for more immediate release upon re-exposure tofluid. The implant was weighed before and after the deposition processto verify the amount of retained ZA. The implants were sterilized usingethylene oxide (EtO), however one skilled in the art will appreciatethat other know methods of sterilization may also be employed.

Assay of ZA Elution. Assays of ZA in solution were achieved using UVspectrophotometry. Bisphosphonates on their own lack a detectablechromophore, making them difficult to assay by simple conventionalanalytical methods. However, the metal chelating properties ofbisphosphonates are well known. When bisphosphonates complex withcertain metal ions, a chromophore with suitable. UV activity forspectrophotometric analysis is created (Ostovic D, Stelmach C, HulshizerB. Pharmaceutical Research. 10:470-472, 1993). The method for assayingZA in aqueous-based solutions is based on the formation of a ZA complexwith iron (III) ions (Kuljanin J, Jankovic I, Nedeljkovic J, PrstojevicD, Marinkovic V. Journal of Pharmaceutical and Biomedical Analysis.28:1215-1220, 2002). Of the three components in the prescription drugZometa™ (ZA, mannitol, and sodium citrate), only ZA forms a complex withiron (III) ions. A standard solution of iron (III) chloride inperchloric acid is added to known concentrations of Zometa™ and theabsorbance is measured at 290 nm to create a calibration curve. A highlyacidic medium is needed to prevent the hydrolysis of the iron (III)ions. Aliquots of the soak solution from the Zometa™-coated implants canbe analyzed by adding the standard iron (III) chloride solution to thesoak solution and measuring the absorbance at 290 nm. By measuring theabsorbance, the concentration of the sample can be calculated and thusthe mass of the drug released from the implant for a given time periodcan be determined. UV absorbance of aqueous solutions of mannitol andsodium citrate indicated that they do not interfere with ZA absorbancewhen iron (III) is used as the chelating agent.

Four porous tantalum implants with and without HA coating were depositedwith 0.05 mg of ZA as described. For each implant group, elutioncharacteristics were ascertained by immersing each implant in a testtube containing 5 ml of 0.9% saline at 3TC. At mutiple test intervals (1min, 3 min, 5 min, 10 min, 15 min, 30 min, 1 hr, 6 hrs, 12 hrs, 1 day, 3days, 7 days, 14 days, 21 days, 42 days, 84 days, and 98 days), theimplant was removed from the test tube, the saline was thoroughly mixed,and aliquots were removed for ZA assays using UV spectrophotometry.After each time interval for each implant, the implant was placed in atest tube with fresh replacement of 5 ml of saline. This elution modelavoided build up of boundary layers and more closely resembled a dynamicsystem. The 84-day assay time corresponded to the 12 week length of thein vivo implant studies. Assays ceased once the total amount of ZA wasreleased.

In Vivo Studies. The previously described canine ulnar intramedullaryimplant model was utilized (study approved by institutional reviewboard) (Bobyn J D, Tanzer M, Harvey E J, Krygier J J, Little D G. J BoneJoint Surg (Br), In press, 2005). The surgical procedure involvedanesthetizing the dog with a general anesthesia and under sterileconditions exposing the proximal ulna. A two-centimeter incision wasmade over the olecranon process and the triceps tendon was split bysharp dissection down to bone. Under fluoroscopic guidance, a 5.0 mmdrill was oriented along the long axis of the ulna and in line with theintramedullary canal. A 5.5 cm long hole was drilled under fluoroscopyto ensure the proper orientation of the drill hole and to preventcortical penetration. The porous implant was then tapped down theintramedullary canal of the ulna with a punch and mallet. The implantwas slightly countersunk to avoid postoperative irritation of theoverlying triceps tendon. The wound was irrigated and closed in astandard fashion. The procedure was repeated on the contralateral side.Positioning of the implants inevitably varied somewhat in terms of depthof insertion within the canal and spatial orientation within the canal.This together with differences in ulna size from animal to animalresulted in variability of the proximity of different regions of eachimplant to endosteal cortical bone. Each dog received either two controlHA coated implants without ZA or two HA coated implants dosed with 0.05mg ZA. Five control dogs and four with ZA-dosed implants were studied at12 weeks. This protocol was utilized instead of one with a control and aZA-dosed implant in each animal because it avoided the possibility ofsystemic absorption of ZA from one side and influence of a controlimplant on the other side. No dogs had any complications or systemicillness related to the ZA administration.

Histological examination. After sacrifice, the ulnae were harvested,stripped of soft tissue, radiographed and processed for undecalcifiedhard-section histology (FIG. 6). This involved dehydration in ascendingsolutions of ethanol, defatting in ether and acetone, and embedding inmethylmethacrylate. Each implant was cut transversely into 6 or 7sections at 7-8 mm intervals. The sections were radiographed andpolished, sputter coated with gold-palladium, and imaged withbackscattered scanning electron microscopy. For each section,computerized image analysis based on grey level discrimination was usedto identify bone and implant and to generate quantitative information onthe extent of bone ingrowth, defined as the percentage of the availableporosity that was filled with new bone. Also tabulated in each sectionwas the number and size of the individual islands of bone within theimplant pores. In sections cut through the ulnar diaphysis, wheredelineation of the endosteal cortex was clearly evident, the total areaof peri-implant bone contained within the intramedullary canal was alsotabulated. In these calculations the implant area (and bone within) wasnot included. For each section, the peri-implant bone was expressed as apercentage of the total peri-implant area within the intramedullarycanal (not including the implant).

Statistical analysis. The quantitative histological data werestatistically analyzed using multiple two-level hierarchical models. Atthe first level of the model, the set of results from the limbs of eachdog was assumed to follow a normal distribution with dog-specific meansand a global variance parameter. At the second level of the model, themeans from each dog in each group (control, ZA-dosed) from the firstlevel followed a second normal distribution, with the mean representingthe overall mean for the treatment or control groups and the variancerepresenting the variability within the group. A similar statisticalmodel was also run where the results for each dog were allowed to varywith the distance (section level) along the ulna. As these results werevirtually identical to those from the model without this extra variable,only the results from the simple hierarchical model are presented. Themean values and differences between means for control and ZA-dosedimplants were estimated with 95% confidence intervals (CI). These dataincluded the amount of peri-implant bone in diaphyseal sectionsexpressed as ‘a percentage of the intramedullary canal size, the overallextent of bone ingrowth, and the number of bone islands within theimplant pores and bone island size.

Results

ZA Elution. The elution experiment indicated very different releasecharacteristics for implants with and without HA coating as shown inFIG. 7. All of the ZA was released from the non-HA coated implant within1 hour, confirming that the ZA did not bind to the implant. However,only about 65% of the ZA was released from the HA-coated implant in thesame time frame. Almost 95% of the ZA was released from the HA-coatedimplant after soaking for 6 weeks, confirming slow release of the boundZA over this time.

Histoloaic examination. A total of 62 histologic sections from 10control implants and 54 sections from 8 ZA-dosed implants were examinedwith contact radiography and backscattered scanning electron microscopy.The contact radiographs revealed varying degrees of peri-implant bonewithin the intramedullary canal in all sections. This bone was almostalways more dense and/or abundant in sections of ZA-dosed implantscompared with control implants (FIG. 8). In some sections of theZA-dosed implants, the peri-implant bone formation was so extensive thatthe intramedullary canal appeared to be virtually obliterated with bone(FIGS. 9, 10). This was most evident in the metaphyseal andmetaphyseal-diaphyseal region of the ulna. However, even the diaphysealregion of the canine ulna, where there is normally fatty tissue andlittle intramedullary bone, demonstrated bone augmentation around theZA-dosed implants (FIG. 10). The backscattered scanning electron imagesrevealed the extent of peri-implant bone more clearly; 24 controlsections from 4 dogs and 24 ZA-dosed sections from 4 dogs were selectedfrom diaphyseal regions for quantification of peri-implant bone. Thesedata are listed in Table 2. The mean percentage filling of theperi-implant space with bone in control sections was 13.8% (95% CI 2.7to 24.5) compared with 32.2% (95% CI 21.7 to 43.0) for ZA-dosedsections. The 18.4% difference of the means was significant (95% CI 3.3to 33.7). In relative terms, there was on average 2.3 times moreperi-implant bone around the ZA-dosed implants compared with controls.

TABLE 2 Peri-implant bone relative to intramedullary canal size by dogand overall (%) *Control Dog # 1 2 3 4 Mean (95% CI) Bone within Canal7.3 14.8 21.6 11.4 13.8 (2.7 to 24.5) †ZA Dog # 1 2 3 4 Mean (95% CI)Bone within Canal 19.8 38.5 30.4 40.0 32.2 (21.7 to 43.0) *24 sections,8 implants †24 sections, 8 implants ZA: zoledronic acid CI = confidenceinterval

New bone formation within the pores of the tantalum implants wasobserved in all sections, to varying degrees (FIGS. 9, 10, 11). Therewas a general tendency for more bone ingrowth at the implant peripherythan in the center. Although there would tend to be less osteogenicstimulus within the implant center, small islands of bone were observedthroughout the implant cross-sections, to varying degrees. Thequantitative histologic data on bone ingrowth are listed in Tables 3 and4 below. The mean extent of bone ingrowth for the 10 control implantswas 12.5% (95% CI 9.9 to 15.1), while the mean extent of bone ingrowthfor the 8 ZA-dosed implants was 19.8% (95% CI 16.9 to 22.6), a 7.3%difference that was significant (95% CI 3.5 to 11.1) and easily noticedupon visual comparison of control and ZA-dosed sections (FIG. 11). Inrelative terms, there was on average 58% more net bone growth intoZA-dosed implants compared with controls.

TABLE 3 Mean extent of bone ingrowth by dog and overall (%) *Control Dog# 1 2 3 4 5 Mean (95% CI) Ingrowth 13.9 11.6 10.0 12.8 14.2 12.5 (9.9 to15.1) †ZA Dog# 1 2 3 4 Mean (95% CI) Ingrowth 16.0 20.3 20.3 22.6 19.8(16.9 to 22.6) *62 sections, 10 implants †54 sections, 8 implants ZA:zoledronic acid CI = confidence interval

TABLE 4 Mean number and size of bone islands by dog and overall *ControlDog # 1 2 3 4 5 Mean (95% CI) # Bone Islands 131 121 152 142 131  135(102 to 168) Bone Island Size (mm²) 0.017 0.014 0.010 0.014 0.016 0.014(0.008 to 0.020) †ZA Dog # 1 2 3 4 Mean (95% CI) # Bone Islands 109 155206 131  150 (113 to 187) Bone Island Size (mm²) 0.033 0.016 0.016 0.0280.023 (0.016 to 0.031) *62 sections, 10 implants †54 sections, 8implants ZA: zoledronic acid CI = confidence interval

There was no statistically significant difference in the mean number ofbone islands within the implant pores between the control group(mean=135, 95% CI 102 to 168) and the ZA-dosed group (mean=150, 95% CI113 to 187). However, the bone islands within the implant pores were onaverage 0.009 mm² larger in the ZA-dosed implants (95% CI 0.001 to0.017). The bone islands in the control implants had a mean size of0.014 mm² (95% CI 0.007 to 0.021) compared with the ZA-dosed implantswhich had a mean size of 0.023 mm² (95% CI 0.016 to 0.030). Thisrepresented a 71% relative difference in the size of the bone islands.

Disscussion

This controlled experiment illustrated very clearly that ZA can beeffectively delivered directly from an HA coated intramedullary implant.While this was elucidated in the context of ulnar implants, the findingseasily extrapolate to any cementless orthopaedic device such as a hipreplacement prosthesis. The locally delivered ZA resulted in a net gainin both peri-implant and intra-implant bone formation. Of the twomeasured regions of net bone formation, the intramedullary canal wasmuch more substantially affected by the ZA-induced alteration of boneremodeling than the pores within the implant. This makes sense in thatthe normal reparative stimulus to reaming the canal would be strongestat the (mechanically disrupted) endosteal surface of the canal andweakest at the center of the canal, where the implant tended to belocated and where there is little native bone. Quantitative measurementof peri-implant bone was confined to histologic sections from thediaphysis, where delineation of the endosteal border was most evident,but the additional bone around ZA-dosed implants, compared withcontrols, was evident radiographically and histologically in thesections from the metaphysis. However, this could not be quantifiedbecause the thin cortices and more extensive cancellous bone of themetaphysis made identification of the intramedullary canal andquantification of peri-implant bone unreliable.

The additional bone with the pores of ZA-dosed implants (mean of 58%)was substantial and could be of value for augmenting mechanicalattachment of the implant and enhancing implant survivorship. Only thebone within the more superficial region of the implant pores would beexpected to contribute to fixation per se; this was the area where morebone tended to form, possibly influenced by the presence of the HAcoating. It is of interest to note that the extent of bone ingrowth inthis study was substantially higher than in a previous study usingsystemically administered ZA, both for control implants and ZA-dosedimplants. This is likely due to differences between the studies: 6additional weeks of implantation, the HA coating, and the local levelsof ZA.

This experiment verified the utility of HA for chemically binding the ZAand delaying its elution over time. Although the optimum ZA release rateis an unknown factor, based on our prior study in which systemicallyinjected ZA (fast exposure) caused marked enhancement of bone ingrowth,it seems logical that a faster, as opposed to slower release rate wouldbe effective. Healing and remodeling start immediately after surgery andrecruitment of osteoclasts occurs early after surgery when remodeling ismost active. In this context it is important to note that ZA bindsirreversibly with bone and that once administered the concentration ofZA in bone changes very little over time (Li E C and Davis L E. ClinicalTherapeutics 25:2669-2708, 2003). It is also important to note from thework of Peter et al that ZA exposure to a bone surface (i.e., endostealbone) results in local persistence of the drug; in other words,diffusion from a local site is very low (Peter B, Gauthier O, GuicheuxJ, Bouler J M, van Lenthe H, Muller R, Zambelli P Y, Pioletti D P:Poster presentation, Trans 7^(th) World Biomat Congress, Sydney,Australia 2004, p 1174). The pilot elution experiment indicated thatabout 60% of ZA on an HA-coated implant was released very early(analogous to systemic injection), followed by a slower release of theremainder. This appeared to be a reasonable elution characteristic forthe purposes of the in vivo studies, although further experimentation inthis area is required for optimization.

According to Li and Davis (Li E C and Davis L E. Clinical Therapeutics25:2669-2708, 2003), at concentrations greater than 2.5×10-10 mol/L ZAcan be toxic to bone by inhibiting osteoblast proliferation and DNAsynthesis. Peter et al (Peter B, Gauthier O, Guicheux J, Bouler J M, vanLenthe H, Muller R, Zambelli P Y, Pioletti D P: Poster presentation,Trans 7^(th) World Biomat Congress, Sydney, Australia 2004, p 1174)recently examined the response of murine (MC3T3) and human (MG-63)osteoblastic cells to ZA and determined that a concentration below 10 μMcan be considered safe for cellular activity. Based on these studies itwas decided that a ZA dose of 0.05 mg should fall well within the saferange. Based on the release rate of ZA in saline solution, an estimateof the canine ulnar bone volume contained within the peri-implant space,the target local bone concentration described in earlier studies (Li E Cand Davis L E. Clinical Therapeutics 25:2669-2708, 2003; Peter B,Gauthier O, Guicheux J, Bouler J M, van Lenthe H, Muller R, Zambelli PY, Pioletti D P: Poster presentation, Trans 7^(th) World BiomatCongress, Sydney, Australia 2004, p 1174) and our earlier ulnar studywith systemic ZA (Bobyn J D, Tanzer M, Harvey E J, Krygier J J, Little DG. J Bone Joint Surg (Br), In press, 2005.), the 0.05 mg dose was alsothought to lie within the biologically effective range. It was clearlysufficient for altering local bone remodeling and causing a net gain inbone formation around and within the porous tantalum implants withoutany histologic evidence of cellular toxicity. Prior to human applicationof this drug delivery concept, further studies would have to beperformed to clarify the minimum effective dose for eliciting anappreciable gain in net local bone formation. It was of interest to notethat the number of bone islands within the pores of control and ZA-dosedimplants did not differ significantly; the same occurred with ourprevious study using systemic ZA at a dose of 0.1 mg/kg (Bobyn J D,Tanzer M, Harvey E J, Krygier J J, Little D G. J Bone Joint Surg (Br),In press, 2005). With both studies, the additional bone within porousimplants exposed to ZA was primarily due to larger bone island size, notincreased number, further supporting the notion that ZA acts bysuppressing catabolic remodeling as opposed to boosting anabolicactivity.

Although the preferred dose of ZA applied to the implant is about 0.05mg, more or less bisphosphonate can be used. Particularly, theexperiments were conducted using total ZA doses of 0.2 mg and 0.4 mg.While increasing the dosage of ZA was found to produce more boneformation around and within the implant, the additional bone formed wasfound to be woven bone, i.e. bone which is more immature relative to thebone formed when using a total dose of 0.05 mg. Thus, such higher dosesof ZA have shown to promote bone for which the maturity is negativelyaffected. Accordingly, and perhaps somewhat counter-intuitively, it hasbeen found that the lower ZA dose of 0.05 mg produces the best resultsof the experiments conducted. A ZA dose of at least less than 0.1 mg istherefore preferred, with a dose of 0.05 mg being the most preferreddose of ZA applied to the implant.

The present bone implant drug delivery system is preferablybiocompatible, mechanically strong, capable of achieving adequate drugloading, simple to fabricate, and unaffected by sterilization. With aporous implant there is the additional consideration that the drugdelivery system does not occlude the pores or hinder bone ingrowth. Inthe cardiac stent industry bioresorbable polymers are utilized, however,with a bioresorbable polymer delivery system there exists the need toidentify the chemical degradation products of the polymer and theireffects on local tissue response. The specific use of HA as animmobilizer of ZA on an orthopaedic implant is not the only possiblemeans to provide modulated, multi-phasic release of a bone stimulatingactive agent to local bone surround the implant, however the preferredembodiment described above is simple, effective, and advantageous giventhe long clinical history of HA use in hip implant design. The conceptproposed does not necessarily require use of a porous implant. HA mayalso be used as an adjuvant fixation on implants without porous surfacetreatments and would be equally effective for binding ZA in theseinstances. The reason for the rapid release of 60% of the ZA was mostlikely due to its presence (availability) on the inner, non-HA coatedregions of the porous tantalum struts.

Therefore conventional orthopaedic implants can be effectively used tolocally deliver pharmaceutical agents to bone for modulation of bonehealing/formation. The net positive remodeling response to a locallyeluted bisphosphonate was consistent and substantial, to an extent thatcould provide clinical benefit to implant stability and fixation. Asignificant advantage of bisphosphonates over bone morphogeneticproteins is their relatively low cost, an important consideration giventhe increasingly stringent global health care constraints. An ancillarybenefit of using bisphosphonates is their documented effect onmitigating the effects of stress shielding and wear particle inducedosteolysis (Shanbhag A S, Hasselman C T, Rubash H E. Clin Orthop344:33-43, 1997; Soininvaara T A, Jurvelin J S, Miettinen H J A,Suomalainen O T, Alhava E M, Kroger P J. Calcified Tissue Int71:472-477, 2002; Venesmaa P K, Kroger H P, Miettinen H J, Jurvelin J S,Suomalainen O T, Alhava E M. J Bone Miner Res 16:2126-2131, 2001;Wilkinson J M, Stockley I, Peel N F, Hamer A J, Elson R A, Barrington NA, Eastell R. J Bone Miner Res 16:556-564, 2001). This concept has wideranging implications for various types of bone devices, arthroplastyimplants being the most obvious but also for fracture fixation, tumorresection, limb lengthening implants, spinal implant, and/or dentalimplants.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth, and as follows in the scopeof the appended claims.

1-48. (canceled)
 49. An implantation method for releasing an activeagent in at least two phases, the method comprising the steps of:providing an implant having a porous portion; coating the porous portionwith a calcium phosphate compound; contacting the implant with abisphosphonate compound to form: (i) a bisphosphonate layer chemicallybound to the calcium phosphate compound at the surface of the porousportion, and (ii) bisphosphonate molecules non-chemically attachedinside pores of the porous portion; implanting the implant at leastpartially within a subject; burst-releasing, in a first release phase,the non-chemically attached bisphosphonate molecules in the subject uponcontact with body fluids of the subject; and slowly releasing, in asecond release phase, the chemically bound bisphosphonate layer in thesubject upon contact with the body fluids.
 50. The method of claim 49,wherein the release of the non-chemically attached bisphosphonatemolecules and the chemically bound bisphosphonate layer in the subjectpromotes bone formation around or within the implant; and wherein thepromoted bone formation is modulated by the first and second releasephases.
 51. The method of claim 49, wherein the implant is at least oneof a joint implant, a spine implant and a dental implant.
 52. The methodof claim 51, wherein the joint implant is one of a hip, knee, elbow,ankle and shoulder implant.
 53. The method of claim 49, wherein theporous portion comprises a biocompatible surface having interconnectingpores formed therein.
 54. The method of claim 53, wherein thebiocompatible surface is selected from the group consisting of asintered bead porous surface, a fiber metal porous surface, a texturedsurface and a plasma spray surface.
 55. The method of claim 49, whereinthe implant is fabricated from a material comprising at least one oftitanium, titanium-based alloy, zirconium, niobium, cobalt-based alloy,tantalum, stainless steel and polymer.
 56. The method of claim 49,wherein the pores are of a size ranging from 20 to 1000 μm.
 57. Themethod of claim 49, wherein the pores are of an average size of 100 to700 μm.
 58. The method of claim 49, wherein the bisphosphonate is abisphosphonate of at least a third generation.
 59. The method of claim58, wherein the bisphosphonate is selected from the group consisting ofbisphosphonate zoledronic acid (ZA), ibandronate and risedronate. 60.The method of claim 59, wherein the bisphosphonate is bisphosphonatezoledronic acid provided on the implant in a dose of less than 0.4 mg.61. The method of claim 60, wherein a dose of less than 0.05 mg of thebisphosphonate zoledronic acid is provided on the implant.
 62. Themethod of claim 58, wherein the bisphosphonate is provided on theimplant in a maximum dose equivalent to a bisphosphonate zoledronic aciddose of about 0.4 mg.
 63. The method of claim 62, wherein the maximumdose is equivalent to a bisphosphonate dose of about 0.05 mg.
 64. Themethod of claim 49, wherein the calcium phosphate compound comprises atleast one of hydroxyapatite, tricalcium phosphate, dicalcium phosphate,amorphous calcium phosphate, and tetracalcium phosphate monoxide. 65.The method of claim 49, wherein the bisphosphonate includes nitrogen.66. A bone implantation method for releasing an active agent in at leasttwo phases, the method comprising the steps of: providing a bone implanthaving a porous portion; coating an outer surface of the porous portionwith a calcium phosphate compound; applying a bisphosphonate compound tothe porous portion to form: (i) a bisphosphonate layer chemically boundto the calcium phosphate compound on the outer surface of the porousportion, and (ii) bisphosphonate compound molecules non-chemicallyattached to the porous portion within pores thereof free of the calciumphosphate compound; implanting the implant at least partially within asubject; releasing, in a first release phase, the non-chemicallyattached bisphosphonate molecules from the implant to promote boneformation around or within the implant; releasing, in a second releasephase, the chemically bound bisphosphonate layer from the implant topromote bone formation around or within the implant; and wherein therelease rate of the first release phase is different from the releaserate of the second release phase.
 67. The method of claim 66, whereinthe promoted bone formation is Modulated by the first and second releasephases.
 68. The method of claim 66, wherein the implant is at least oneof a joint implant, a spine implant and a dental implant.
 69. The methodof claim 68, wherein the joint implant is one of a hip, knee, elbow,ankle and shoulder implant.
 70. The method of claim 66, wherein theporous portion comprises a biocompatible surface having interconnectingpores formed therein.
 71. The method of claim 70, wherein thebiocompatible surface is selected from the group consisting of asintered bead porous surface, a fiber metal porous surface, a texturedsurface and a plasma spray surface.
 72. The method of claim 66, whereinthe implant is fabricated from a material comprising at least one oftitanium, titanium-based alloy, zirconium, niobium, cobalt-based alloy,tantalum, stainless steel and polymer.
 73. The method of claim 66,wherein the pores are of a size ranging from 20 to 1000 μm.
 74. Themethod of claim 66, wherein the pores are of an average size of 100 to700 μm.
 75. The method of claim 66, wherein the bisphosphonate is abisphosphonate of at least a third generation.
 76. The method of claim75, wherein the bisphosphonate is selected from the group consisting ofbisphosphonate zoledronic acid (ZA), ibandronate and risedronate. 77.The method of claim 76, wherein the bisphosphonate is bisphosphonatezoledronic acid provided on the implant in a dose of less than 0.4 mg.78. The method of claim 77, wherein a dose of less than 0.05 mg of thebisphosphonate zoledronic acid is provided on the implant.
 79. Themethod of claim 75, wherein the bisphosphonate is provided on theimplant in a maximum dose equivalent to a bisphosphonate zoledronic aciddose of about 0.4 mg.
 80. The method of claim 79, wherein the maximumdose is equivalent to a bisphosphonate dose of about 0.05 mg.
 81. Themethod of claim 66, wherein the calcium phosphate compound comprises atleast one of hydroxyapatite, tricalcium phosphate, dicalcium phosphate,amorphous calcium phosphate, and tetracalcium phosphate monoxide. 82.The method of claim 66, wherein the chemically bound bisphosphonatelayer is slowly released in the subject and the non-chemically attachedbisphosphonate molecules are released more quickly in the subject. 83.The method of claim 66, wherein the bisphosphonate includes nitrogen.84. A bone implantation method for releasing an active agent, the methodcomprising the steps of: providing a biocompatible bone implant;selectively coating the bone implant with a calcium phosphate compoundto form: (i) at least a first region with a calcium phosphate coatingthereon, and (ii) at least a second region free of the calciumphosphate; applying a bisphosphonate compound to the bone implant toform: (i) a bisphosphonate layer chemically bound to the calciumphosphate coating of the first region, and (ii) bisphosphonate moleculesnon-chemically attached to the second region of the bone implant;implanting the implant at least partially within a subject; releasingthe bisphosphonate from the first and second regions at different ratesafter implanting the implant.
 85. The method of claim 84, wherein thenon-chemically attached bisphosphonate molecules are released in thesubject at a rate greater than that of the chemically boundbisphosphonate layer.
 86. The method of claim 84, wherein the release ofthe non-chemically attached bisphosphonate molecules and the chemicallybound bisphosphonate layer in the subject promotes bone formation aroundor within the implant; and wherein the promoted bone formation ismodulated by the different release rates of the bisphosphonate in thesubject.
 87. A bone implantation method for releasing a material in atleast two phases, the method comprising the steps of: providing abiocompatible bone implant; applying a binding agent to the bone implantto form: (i) at least a first region with applied binding agent, and(ii) at least a second region free of the binding agent; applying a bonegrowth stimulating active agent to the bone implant to form: (i) anactive agent layer chemically bound to the applied binding agent of thefirst region, and (ii) active agent molecules non-chemically attached tothe second region of the bone implant; implanting the implant at leastpartially within a subject; rapidly releasing, in a first release phase,the non-chemically attached active agent molecules of the second regionto bone of the subject proximate the implant to locally promote boneformation around or within the implant; gradually releasing, in a secondrelease phase, the chemically bound active agent layer of the firstregion to bone of the subject proximate the implant to locally promotebone formation around or within the implant; wherein the release rate ofthe first release phase is different from the release rate of the secondrelease phase; and wherein the promoted bone formation is modulated bythe first and second release phases.
 88. The method of claim 87, whereinthe active agent is a bisphosphonate and the binding agent is a calciumphosphate compound.
 89. The method of claim 88, wherein thebisphosphonate is at least a third generation bisphosphonate.
 90. Themethod of claim 87, wherein the bone implant includes a porous portionand has an outer surface defining the first region and internal poresdefining the second region.
 91. The method of claim 88, wherein thecalcium phosphate compound comprises at least one of hydroxyapatite,tricalcium phosphate, dicalcium phosphate, amorphous calcium phosphate,and tetracalcium phosphate monoxide.